This chapter discusses the role of climate variability and change and their effects on the marine environment. As the frequency of physical forcing increases, the biological changes progress from local effects on individuals at synoptic weather scales, towards regional effects on population dynamics at monthly to decadal scales, and over and across basins on systems ecology at multidecadal timescales and longer. The nature of the impact is size- and age-dependent with generally greater and more rapid impacts on the smaller and younger individuals. The use of large-scale climate indices to link climate forcing with ecological responses is highlighted as are the insights gained through comparative studies between ecosystems or between fish populations that inhabit different hydrographic regimes.